1. Field of the Invention
This invention relates to a method of processing a radiation image in a radiation image recording system used for medical diagnosis and apparatus therefor. This invention particularly relates to an image gradation processing method and apparatus in a radiation image recording system in which a stimulable phosphor is used for recording radiation image information as an intermediate recording medium and the image recorded therein is read out for reproduction and finally recorded on a recording medium.
2. Description of the Prior Art
For medical diagnosis, there are generally used X-ray films for taking radiographs. Since the X-rays do harm to the human body, it is impossible from the viewpoint of safety to expose the human body to X-rays of high dose. Therefore, it is desired that the necessary information in the radiograph can be obtained by exposing the human body only once to the X-ray of comparatively low dose. On the other hand, radiographs should preferably have both a wide exposure latitude and high image quality of high contrast, high sharpness, low noise etc. for viewing and diagnosis purposes. Unfortunately, since the conventional radiography is designed so as to satisfy all the required conditions to some extent, the range of recording density or the ability to record various levels of information and the image quality are both insufficient and neither of these properties are completely satisfied.
In view of the above circumstances, the inventors of this invention suggested in Japanese Pat. application No. 84741/1978 a radiation image recording system in which radiation image information is recorded in a stimulable phospher by exposing the phosphor to a radiation transmitting through an object like a human body, then the recorded information is read out by use of a photodetector by stimulating the phosphor with a stimulating ray like a laser beam or the like, and the read out information is recorded on a recording medium by modulating a recording laser beam or the like with the information read out by the photodetector.
This radiation image recording system is far more advantageous that the conventional radiography using a silver halide photographic film in that the image is recorded over a very wide dynamic range of radiation exposure. Therefore, this system is particularly valuable when it is applied to the radiographic diagnosis of the human body. By using this system for such purposes, it becomes possible to obtain the information which cannot be obtained in the conventional radiography because of the limited exposure range that can be recorded and observed on the conventional X-ray film.
Further, it was also made possible in the above method that the light emitted by the stimulable phosphor upon stimulation is converted to an electric signal and processed to improve the image quality for viewing and diagnosis purposes. In the above patent application, however, the concrete method for improving the image quality to enhance the diagnostic efficiency and accuracy is not definitely disclosed.
Then, the present inventors have conducted researches and investigations for enhancing the diagnostic efficiency and accuracy and further suggested in Japanese patent application No. 163571/1978 an unsharp masking process for emphasizing the super-low frequency component and in U.S. patent application No. 106,733 an image gradation processing method for modifying the gradation of a radiation image of the chest.
The present invention is related to the image gradation processing of a radiation image, particularly to the method of and apparatus for gradation processing a radiation image of the chest.
In the conventional chest radiography, an X-ray film is used for recording the X-ray transmission image of the chest and recorded image is observed with the naked eyes for diagnosis. In the chest radiography, there are recorded lungs, a heart and a spine. The spine has the lowest density since the transmittivity thereof to the X-rays is low. The heart has the second lowest density since the transmittivity thereof to the X-rays is comparatively low. The lungs have high density since the transmittivity thereof to the X-rays is high. Further, since the lungs have complex trachea/bronchus and blood vessels, the image of the lungs is very complicated. The part outside the substantial image of the human body has the upmost density since this part of the X-ray film is exposed to X-rays directly coming from the X-ray source.
As mentioned above, the chest radiography has various information of various parts of the human body which is recorded in the density having a wide range of levels. Sometimes, the density ranges from 0 to 3.5 in terms of optical density. Further, since the various parts are not recorded in the desirable contrast respectively, it is very difficult and necessary to have a great skill to make proper diagnosis from the radiograph in which the disease must be found out from a very slight variation in density in the image.
It is generally known in the art that the image properties can be changed by processing the image by use of an electronic signal or information processing method. For instance, even in radiography, it is possible to read the image recorded on the X-ray film by an optical scanning means and process the read out signal by a signal processing means to change the various image properties such as contrast and the density level and then record a visible image on a recording film or the like based on the processed signal.
In the radiography, however, the recorded image is used for the purpose of "diagnosis" and the diagnostic efficiency and accuracy (the level of easiness for diagnosis or adaptability to diagnosis) are not simply enhanced by simply making so-called "good" image from the point of the ordinary image quality factors such as sharpness, granularity and contrast. Rather than these factors, the diagnostic efficiency and accuracy are influenced by other complex factors such as reference with the normal shade, reference with the anatomical structure and utilization of other diagnostic view or records.
In view of the above circumstances, the present inventors further conducted researches and investigations as to the image gradation processing for enhancing the diagnostic efficiency and accuracy, and found that the diagnostic efficiency and accuracy was improved in the radiation image of the chest by lowering the level of the signal corresponding to the between the densities of the heart and the lungs of the radiation image to lower the contrast of the heart and thereby raising the contrast of the lungs. This method is disclosed in U.S. Pat. application No. 106,733.
According to this method, the diagnostic efficiency and accuracy of the radiation image of the chest was markedly enhanced. As a result of further researches and investigations, it has been discovered that the diagnostic efficiency and accuracy of the radiation image of the chest are further improved by lowering the level of the signal corresponding to the density between the densities of the spine and the heart.